Stents are generally tubular devices that are used to support a segment of blood vessel or other anatomical lumen so as to maintain its patency. Stents are useful, for example, in the treatment of atherosclerotic stenoses in blood vessels, maintaining blood perfusion to downstream tissue after opening of a flow restriction.
Various types of stent designs have been developed for treating diseases of blood vessels and other tubular structures inside the body. The currently available stents can be classified into two broad categories: balloon-expandable and self-expanding.
A balloon-expandable stent is collapsed down onto a folded balloon on the end of a balloon dilatation catheter. The stent maintains this collapsed configuration until it is affirmatively expanded. When the stent has been properly positioned within the lumen, the balloon within the stent is inflated to an appropriate size, expanding the stent to the desired diameter. The balloon is then deflated, and the catheter is withdrawn, leaving the expanded stent in place within the lumen. The stent remains in its expanded state because of the plastic deformation that was imparted to its structural elements during expansion.
A balloon-expandable stent has many attractive attributes. Its diameter and outward force to the vessel wall can be adjusted by controlling the inflation pressure of the balloon. Also, after deployment the stent is a semi-rigid structure that can conform to some extent longitudinally, but maintains a rigid scaffolding that prevents vessel collapse in the radial direction.
However, balloon-expandable stents also present certain disadvantages. One such disadvantage is that there is typically some component of elastic recoil after expansion. This elastic recoil usually means that there is a reduction in diameter after the balloon is deflated. The degree of reduction in diameter is related to the material selection, structural design, and degree of inward force from the vessel wall. These factors vary from stent to stent and from situation to situation, presenting a challenge for the practitioner to achieve the desired outcome in repeatable manner.
In contrast to the balloon-expandable stent, a self-expanding stent is formed to assume a pre-determined diameter. The stent is radially compressed and placed on the end of a delivery catheter. Some means, such as a surrounding sheath, must be provided to maintain the stent in its compressed state on the end of the delivery catheter as it is delivered to the target site within a lumen in the body of a patient. When the sheath is retracted, the stent recovers to its pre-determined diameter through a shape memory effect.
This type of stent operates in an entirely elastic mode, so it is possible for this stent to be more flexible than its balloon-expandable counterpart. For this reason, it is possible to create self-expanding stents with more tightly arranged mesh patterns, without resulting in an axially rigid stent. Because of these attributes, self-expanding stents are particularly useful in larger vessels, where superior conformability and vessel wall coverage are relatively more important. It is also the stent of choice whenever there is a concern that the stent could be crushed due to body movement or external pressure, because this type of stent will elastically recover after temporary collapse, whereas a balloon-expandable stent will not.
The disadvantages of self-expanding stents are mainly that the diameter of the stent is not as adjustable as with a balloon-expandable stent. For instance, if the vessel size is too small relative to the stent rest diameter, then a self-expanding stent will exert a radially outward force on the vessel only until the stent reaches its relaxed diameter. If the vessel size is too large for the stent, the stent cannot be adjusted to fit the vessel, and it will not be affixed to the vessel wall.
Balloon-expandable and self-expanding stents are known that employ ratcheting or latching means for retaining the expanded configuration. One purported benefit of stent designs that contain latching elements is the capability for more precise lumen sizing. In the balloon-expandable, latching stent designs, a latch allows radial expansion, but limits post deployment reduction in diameter. In the self-expanding case, a latch can be employed to prevent over-expansion, and provides an upper limit to the chronic outward force on the vessel.
However, a disadvantage of stents with latching mechanisms is that the latches contribute significantly to the bulk of the stent. For this reason known stents with latching mechanisms have exhibited reduced flexibility and larger undeployed profile, i.e., diameter. These characteristics are important because they relate to the ability of the stent to be delivered to the desired target site. The flexibility of the stent relates to how well it will navigate turns in the vessel, and the diameter of the stent determines the minimum diameter in the vessel that can be traversed by the stent as it is being delivered to the target site. Additionally a larger profile stent requires that larger accessory devices be used to introduce the device. The need for larger accessory devices means that the puncture in the vessel wall for introducing the stent needs to be larger as well, leading to longer post-procedure patient recovery times.
Another drawback to known stents with latching mechanisms is that the sizing of the stent is not continuous but rather ratchets in discrete increments. The sizing increment that is available to the user is typically a function of the size and spacing between latches. So fine adjustment of the stent diameter in its expanded state is restricted when compared to non-latching, balloon-expandable stents. This effect becomes more significant as the size of the target vessel becomes smaller, and so the use of the previously proposed ratcheting stents are practical only in larger, non-coronary vessels.